\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{circuitikz}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes}
\ctikzset{bipoles/length=1cm}
\begin{document}
\small
\begin{circuitikz}[>=latex, scale=1,european]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw (0,0) to [R=$R_1$] (3,0)to [R=$R_2$] (5,0)to [R=$R_3$] (8,0);
  \draw [fill=white](0,0) circle (2pt);\draw [fill=white](8,0) circle (2pt);
  \draw [<->](0,-.2)--(0,-4)--(8,-4);
  \draw [very thick] (0,-.8)node[left]{$U$}--(1.1,-.8)--(1.9,-1.5)--(3.6,-1.5)--(4.4, -2.8)--(6.1,-2.8)--(6.9,-4);
  \foreach \x in {1.1,1.9,3.6,4.4,6.1,6.9}
  {
      \draw [dashed](\x, -.25)--(\x, -4);
  }
  \draw [<->|](1.1,-.8)--node [left]{$U_1$}(1.1,-1.5);
  \draw [<->|](3.6,-1.5)--node [left]{$U_2$}(3.6,-2.8);
  \draw [<->|](6.1,-2.8)--node [left]{$U_3$}(6.1,-4);
  \draw [dashed](1.1, -1.5)--(1.9, -1.5);
  \draw [dashed](3.6, -2.8)--(4.4, -2.8);
\end{circuitikz}
\end{document}